The Distances to Open Clusters from Main-Sequence Fitting. III. Improved Accuracy with Empirically Calibrated Isochrones

We continue our series of papers on open cluster distances with a critical assessment of the accuracy of main-sequence fitting using isochrones that employ empirical corrections to the color-temperature relations. We use four nearby open clusters with multicolor photometry and accurate metallicities and present a new metallicity for Praesepe ([Fe/H]=+0.11±0.03) from high-resolution spectra. The internal precision of distance estimates is about a factor of 5 better than the case without the color calibrations. After taking into account all major systematic errors, we obtain distances accurate to about 2%–3% when there exists a good metallicity estimate. Metallicities accurate to better than 0.1 dex may be obtained from BVICKs photometry alone. We also derive a helium abundance for the Pleiades of Y=0.279±0.015, which is equal within the errors to the Sun's initial helium abundance and that of the Hyades. Our best estimates of distances are (m-M)0=6.33±0.04,8.03±0.04, and 9.61±0.03 to Praesepe, NGC 2516, and M67, respectively. Our Pleiades distance at the spectroscopic metallicity,(m-M)0=5.66±0.01(internal)±0.05(systematic), is in excellent agreement with several geometric distance measurements. We have made calibrated isochrones for -0.3≤[Fe/H]≤+0.2 available online.